Power factor correction capacitors and transformers are widely used on electrical transmission and electrical distribution systems. These devices incorporate a tank-like structure for confining a dielectric about an electrical component through which electrical current flows. Good engineering requires that the designer consider the probability that these devices will fail in service.
In the case of a power factor correction capacitor, tank rupture will occur if the total energy applied to the capacitor under failure conditions (e.g. dielectric breakdown, internal short-circuiting, etc.) is greater than the ability of the tank structure to contain such energy. The energy applied can occur under a wide variety of current-time conditions ranging from currents modestly in excess of normal ratings for periods of days, weeks, months or longer, to very high currents over a very short period of time, such as would occur during the dumping of energy stored in a parallel set of capacitors in a large shunt bank into a failed capacitor unit. The specific cause of tank rupture is internal pressure, either isobaric or localized, which is sufficient to stretch the structural members which form the capacitor tank beyond their ultimate strength. The increase in internal pressure is caused by self-generated heat losses and by arcing; rupture times less than 10 minutes are caused primarily by internal arcing (Standard Handbook for Electrical Engineers, by D. G. Fink, 11th Edition, Chapter 10, Article 276). The amount of arcing, in turn, is determined by the location of the failure, the construction of the capacitor and the magnitude and duration of the fault current.
In the event of failure of the capacitor, the normal protective device--typically a fuse--should disconnect the capacitor from the energized circuit prior to rupturing (e.g. the venting of vapors, liquids and/or solids which may or may not be burning at the time of expulsion) the capacitor enclosure or tank. Unfortunately, fuse selection involves much of the same engineering judgement that goes into the design, manufacture and service application of capacitors; therefore, it is possible for a capacitor to rupture or fail when it is placed in service despite the use of fuse protection.
The most commonly used--and lowest cost--fuse type is the expulsion fuse consisting of a fusible link in an expulsion tube. These are available from several manufacturers in a wide range of current ratings, voltage ratings, and fusible elements.
Effectiveness of fuse protection is usually studied by superposition of the expulsion fuse characteristic upon the capacitor characteristic. The conventional view is that if the fuse maximum I.sup.2 t "let through characteristic" is everywhere less than the capacitor "withstand characteristic," the capacitor can be protected from rupture by that fuse. If there is an intersection of the two characteristics, and if the capacitor-fuse combination is expected to operate under the condition prescribed by that intersection, then the probability of that fuse preventing rupture is low.
The protective characteristic of an expulsion fuse is a plot of the Maximum I.sup.2 t Let-Through vs. Symmetric Fault Current. In the low current region, the I.sup.2 t characteristic approaches infinity as the steady state current capability is approached. The minimum I.sup.2 t value is determined by the fusible element size and type. It also takes into account variabilities in performance caused by manufacturing tolerances as well. In the higher current range, I.sup.2 t increases directly as the square of the magnitude of the fault current (i.e. virtually independent of the time during which the fault current flows).
Two other fuses, the vacuum fuse and the chemical fuse, have characteristics similar in shape to those of the expulsion fuse family. A current limiting fuse has a different protective characteristic. The major difference in the shape of the current limiting characteristic in comparison to that of the expulsion fuse is in the high current region where a maximum, approximately constant value of I.sup.2 t is observed.
In today's marketplace there are two types of capacitor dielectric systems--the paper/film/fluid and the All-Film/fluid. (See J. Lapp, "A New Dielectric Fluid for Power Capacitors," Pennsylvania Electric Association, Pittsburgh, PA., Oct. 6, 1976 [McGraw-Edison Bulletin 77016]). Both are impregnated with insulating fluids of various types. The two systems have markedly different rupture characteristics, essentially independent of the type of insulating fluid used. These differences are due to the gassing behavior of a failing paper/film unit in the low current region compared to the non-gassing behavior of a failing All-Film unit in the same current range.
During a progressive failure in a paper/film design an electrical puncture (e.g. arc) of the film/paper/film pack will raise the temperature of the materials in the immediate vicinity of the discharge. This increases the film's solubility in the liquid dielectric and exposes the paper to the arc, thereby permitting the generation of gases during the subsequent decomposition of the paper and the fluid. Continued arcing will cause sufficient gas to be produced to cause the tank to swell. This, in turn, lowers the fluid level, possibly exposing the tops of the packs, and increasing the likelihood of internal flashover and subsequent generation of large quantities of gas.
The failure of one pack will cause an increase in the voltage stress across the remaining series pack groups. The continuation of arcing, swelling, and pack overvoltage will cause the failure to progress, further increasing the current level, and therefore causing more arcing, more gassing, etc. If the capacitor is not disconnected from the system sufficient gas can be generated to cause the tank to rupture.
The lowest currents which may be encountered during the failure of a single pack of a capacitor, range from 111% of the rated current for a 12-series group design to 200% of rated current for a 2-series group design. If alternate internal connections are considered, a single pack failure may result in currents as low as 105% of the rated current. Fusing to prevent rupture under these conditions is extremely difficult unless there is progressive failure and increasing currents. Thus, because of the possible erratic behavior of the paper/film system under failure conditions, the characteristic for the paper/film unit can only be considered approximate.
When an All-Film unit (of the type described by U.S. Pat. No. 3,746,953) is caused to fail under these same conditions, the liquid and the film in the immediate vicinity of the discharge increase in temperature. This causes the solubility of the film in the fluid to increase and allows the deformed foil electrodes to make intimate contact, thereby shorting out the affected capacitor pack and thereby eliminating the arc. With no arc, no gas will be generated; with no gas generated there will be no swelling; and with no swelling there will be no possibility of rupture. Although the shorted pack will cause the voltage stress across the remaining series pack groups to increase, increasing the probability of their failure, gas will not be generated in appreciable quantities unless there is an internal arc. Experience has shown that an arc will not occur in the All-Film capacitor until after all of the series groups have been shorted. Thus, good fusing practices should permit a failing All-Film capacitor to be disconnected from the distribution system long before sufficient gas has been generated to cause tank rupture.
A second type of internal failure consists of the puncture of the capacitor tank insulation resulting in a pack-to-tank arc. This type of failure is rare. There may be, however, a higher probability of this phenomenon occurring in a partially or fully failed paper/film unit aided and abetted by extensive decomposition of other internal materials than there is for the All-Film capacitor, when the capacitor tank is connected to ground and the distribution system neutral is isolated.
In summary, both the expulsion fuse (for symmetric current values less than that defined by the intersection of the two characteristics) and the current-limiting fuse (through the entire current range) can be relied upon to prevent capacitor tank rupture for All-Film units. This is not true for paper/film units. No known fuse--or combination of fuses--can reliably prevent the paper/film units from rupturing under the low-current and internal failure-fault conditions. This is particularly true when partial failure of the capacitor may involve currents only a few percent more than the rated current. Fuses are not normally responsive to such a small increase in current and therefore will not melt or clear the fault under these conditions. Thus, means other than fuses must be used to prevent capacitor tank rupture.
Various means have been devised for venting or releasing the build up of pressure within the pressure vessel or container which houses the capacitor elements or plates. Sprague (U.S. Pat. No. 2,005,055) describes a device incorporating a semi-permeable gasket to control the escape of gases. Peace (U.S. Pat. No. 3,197,547) describes a construction similar to that described by Sprague.
Collins (U.S. Pat. No. 2,199,519) noted that in some types of electrolytic condensers (particularly in dry electrolytic condensers which have a lower heat capacity and a poorer heat dissipation than wet electrolytic condensers), conditions may arise which cause such a large amount of gas to be liberated that a semi-permeable gasket is unacceptable. Collins disclosed an emergency venting means comprising a weakened portion of a condenser container which is pierced by a pressure of a predetermined value so as to prevent the container from exploding. Steele (U.S. Pat. No. 3,401,314) uses a cover member incorporating a venting means similar to that of Collins. Moresi (U.S. Pat. No. 3,204,156) provides a capacitor with a pressure release vent in the form of a longitudinal or axial cut or slot in a portion of the inner wall of the capacitor can.
Myers (U.S. Pat. No. 2,235,778) observed, over many years, that capacitors used in induction heating service failed (not because of overload with associated general excess temperature) by sudden, relatively unpredictable deterioration of a small area of capacitor dielectric. Although the deterioration of the dielectric results in an arc and a nominal short circuit, the nature of construction of many capacitors is such that the fault current is less than the rating of the fuse which is selected to carry normal current to the capacitor. In particular, a small fault of this kind can persist long enough to decompose sufficient dielectric material to create disruptive gas pressures. Myers concluded that unless the capacitor is made for relatively low-volt ampere ratings, fuse protection of the thermal or overcurrent type is very unsatisfactory. Myers solved this problem by using a "fault detector" incorporating a bellows actuated switch.
U.S. Pat. No. 3,772,624 to Keogh summarizes the conventional approach to controlling the pressure within a sealed capacitor or transformer. Most of the earlier devices (he notes) were of the frangible disk type. Keogh used a spring-calibrated, automatic restoring pressure relief. More recently vanGils (U.S. Pat. No. 4,245,277) used a pressure relief valve to control the gas pressure inside a capacitor.
Another approach to the persistent problem of tank overpressurization control was taken by Flanagan in U.S. Pat. No. 4,106,068. Flanagan used the teachings of Rayno (U.S. Pat. No. 3,377,510) to use the outward bulging of a capacitor cover (brought about by increased internal pressure) to provide separation of a pluraity of tabs or leads electrically joining the interior with the exterior of the capacitor. Finally, Sanvito (U.S. Pat. No. 4,240,126) proposed a device which interrupts the electrical circuit joined to the capacitor if a dangerous pressure is reached inside the casing. In passing, Sanvito noted that it has also been proposed to use wires with a weakened section (i.e. thin silver wires, or different types of insertion connectors, etc.) as the interruption member of the electrical circuit (e.g. somewhat analogous to a fuse).
It will be readily apparent by studying the foregoing patents, that the problem of pressure build-up in a capacitor has not been solved. Moreover, it is clear that there is a long felt need for a solution to the problem of capacitor tank construction in view of the failure of so many others. The various approaches that have been taken to control the build-up of pressure, for the most part, employ components that are relatively costly to manufacture and difficult to construct, considering the relatively close tolerances that must be maintained to insure that the protective device will actuate and that excess pressure will not develop. Thus, current solutions are not entirely satisfactory. Significantly, those skilled in the art have heretofore neglected other important aspects of the problem of capacitor tank rupture.
Consider first the basic premise that, at some time, there will be a capacitor rupture or failure. Based on some direct experience with conventional capacitor tanks which have ruptured, it has been the observation of this inventor that the general location of rupture, under certain circumstances, can be predicted (i.e. "Shunt Capacitor Tank Rupture Considerations", by L. M. Burrage, paper A76 043-0 IEEE Power Engineering Society Winter Meeting, printed in January, 1976; "Shunt Capacitor Rupture Prevention--Large Bank Applications", by L. M. Burrage, paper A76 366-5 IEEE Power Engineering Society printed on Apr. 26, 1976; and "Capacitor Tank Rupture Prevention--I.sup.2 T Considerations", by L. M. Burrage, IEEE Transactions--Power Apparatus and Systems Vol. 97, No. 2, Mar/Apr 1978, p. 384-391). For example, in the 300 to 400 amp range (for an All-Film type capacitor) bushing fracture is more likely to occur. This failure is caused by melting and arcing of the internal bushing lead, or by melting of the solder connection at the top bushing stud seal. For moderate currents (e.g., in the range of 1600 amps), the All-Film capacitor tank characteristically ruptures along a bottom side seam if not properly fused. As the current gets higher, more of the bottom seams open, allowing the solids to be expelled. At higher currents (e.g. 4000-8000 amperes), both top and bottom seams are likely to rupture, with the likelihood that the top or upper end will open or rupture increasing as the current increases.
However, both the location and extent of the rupture will vary with the conditions which bring about the failure. For example, under certain conditions, extremely violent rupture can occur wherein pieces of porcelain or shrapnel-like pieces of the capacitor tank can be blown out or expelled at high velocity. Bushing fractures normally occur because of the following sequence of events: extra heat pressurizes the capacitor tank causing it to swell and thereby increasing its volume; this causes the liquid level to drop below the top of the bushing whereupon the internal bushing lead wire is exposed to a vapor rather than being immersed in the dielectric fluid; thus, the bushing lead heats causing even more energy to be released into the interior of the tank and ultimately resulting in thermal fracture of the porcelain bushing. Bushing fracture can also occur at extremely high fault currents which cause the bushing lead wire to literally explode.
Therefore, it is believed that, in the design of a protective device for a tank of the type used to confine an dielectric fluid about an electrical component, three factors must be considered. The two most important conditions to be met by such a device are: the elimination of the likelihood of ejection of shrapnel-like pieces of metal or ceramic; and the retention of most, if not all, of the liquid and solid matter within the tank. If the solids are retained, the likelihood of burning components being expelled and thereby starting secondary fires will be markedly reduced, if not completely eliminated. The last factor of importance is to control the direction of venting in the event that an overpressure condition develops within the tank. Preferably, the venting should be directed away from adjacent components or devices operated in conjunction with the housed electrical component. This will minimize the secondary effects of tank rupture or failure. Of course, the orientation of the tank and, in particular, how and where it is mounted, affects the preferred direction of venting. Heretofore, these three important design considerations have not been completely addressed by designers of tanks for capacitors and other electrical components.
Thus, it should be appreciated that the conventional approach taken to protect against the rupture and damage resulting from a failed tank or enclosure filled with electrolyte involves much beyond that of providing a rupture disk or blowout gasket and selecting a fuse for the service conditions likely to be anticipated. What is needed, is an innovative approach to this persistent problem, one that is easy to manufacture and construct, and a solution which incorporates the best knowledge currently available concerning the manner in which modern capacitor tanks and related structures fail. In particular, a tank design or method of construction which can be readily incorporated into the fabrication of conventional power factor correction capacitor tank enclosures would be readily accepted by the industry and would go far to improve the overall safety and reliability of electrical distribution system components and systems.